The present invention relates to power supplies and more particularly concerns off-line, standby power supplies.
Development and application of computer technology has created a demand for a variety of power protection devices ranging from high isolation transformers to large scale uninterruptible power supply systems. For many years, the basic system for providing a buffered or protected AC power supply was a simple motor generator set. An example of such a buffered supply is the power conditioning system employing a motor generator set that is sold by the assignee of the present invention under the name POWERBLOC. Use of a motor generator set has proven to be a reliable method of protecting a computer from harmful effects of voltage fluctuations and brief power outages. The motor generator set provides isolation of the load from electrical noise and power protection during outages up to about five seconds in duration. However, for those systems, including many large scale computers, that require protection from power outages of greater duration, an alternate or emergency power supply is required to provide power for a period long enough to generate an alarm and allow the operator to shut down or otherwise protect the load from power loss. Various types of emergency power or uninterruptible power supply systems have been employed, but these commonly require on-line operation of all or at least major portions of the auxiliary system. For example, a system has been employed in which a rectifier and inverter are series connected in the main power supply to the motor generator that drives the load, with the inverter being operable by an auxiliary battery for emergency power. The relatively low efficiencies of the inverter and rectifier, which must operate continuously in such prior arrangement, are wasteful of energy. The on-line system is more costly and has descreased reliability because of its required continuous operation. Even an off-line, standby inverter is inefficient and wasteful of energy if it must operate during normal power operation.
Installation of a power conditioning system having an uninterruptible power supply of a size sufficient for a given load requirement may be a factor limiting expansion of the load. An increase in the power required by the load (by the addition of other computers, for example) may require the uninterruptible supply system to provide power beyond its capacity. Therefore, increased load may require use of a larger substitute or an additional uninterruptible supply system with significantly increased cost, inefficiency and reliability problems. Frequently, an original, relatively smaller uninterruptible power supply must be replaced with a larger system to allow expansion of load power requirement. There are no known emergency standby power systems that can be readily and inexpensively retrofitted to existing motor generator power conditioning systems, nor which operate as true standby systems wherein a normally nonoperating inverter is driven only during main line power outage.
Accordingly, it is an object of the present invention to provide an emergency power system that avoids or minimizes above-mentioned problems.